1. Field of the Invention
The present invention relates to solar cells and methods for manufacturing the same.
2. Description of the Related Art
Thin film solar cells in which a compound semiconductor thin film made of Group Ib elements, Group IIIb elements, and Group VIb elements (chalcopyrite compound semiconductor thin film) is used as the light absorption layer have been reported. For example, CuInSe2 (hereinafter, also referred to as “CIS”) or Cu(In,Ga)Se2 in which Ga is dissolved in CIS (hereinafter, also referred to as “CIGS”) are examples of one type of compound semiconductor. Thin film solar cells employing CIS or CIGS as the light absorption layer (hereinafter, also referred to as “CIS solar cells”) are known to have excellent properties, such as a high energy conversion efficiency and no deterioration of the conversion efficiency due to light emission, for example.
A ZnO layer was typically employed as a window layer in conventional high-efficiency CIS solar cells. In recent years, however, attention has turned to solar cells that employ a layer including Zn, Mg, and O (Zn1·x′Mgx′O layer) instead of a ZnO layer, so that the conduction band offset between the light absorption layer and the window layer is closer to an optimal value. Research is also being conducted on CIS solar cells in which a Zn1·x′Mgx′O layer is adopted so as to allow the buffer layer heretofore employed in conventional solar cells to be omitted. Such a solar cell is disclosed in U.S. Pat. No. 6,259,016 and JP 2000-323733A, for example.
However, when a Zn1·x′Mgx′O layer is achieved simply by adding Mg to a ZnO layer, there were cases in which the volume resistivity of the Z1·x′Mgx′O layer became larger than the ZnO layer, lowering the overall electrical conductivity of the solar cell. Currently there is a need for solar cells to be made more efficient, and likewise, there is also a need to further improve the efficiency of solar cells employing a Zn1·x′Mgx′O layer.
In light of the foregoing situation, it is an object of the present invention to provide solar cells that include a layer comprising Zn, Mg, and O and that can be made more efficient than was the case conventionally, and methods for manufacturing such solar cells.